1. Field of the Invention
The invention relates to a dryer for material webs, with exhaust gas recirculation.
The published European Patent Document EP 0 723 126 A1 has heretofore disclosed a control device for a continuous drying process of an industrial dryer. Solvent-laden air circulates in a continuous-flow dryer with progressive heating. A process is disclosed for the optimal control of the solvent-laden air. The condensation of solvents and various products based on solvent compounds is effectively reduced or even prevented. In addition to the reduction of condensation, a more intensive and more uniform mixing of the atmosphere within the dryer is achieved, by which the reliability is increased and the drying process is simplified because areas with a high solvent concentration are reduced. Ambient air is heated within the dryer and mixed with the solvent-laden air. The mixed air is supplied to a first zone of the dryer. In this construction, an open flame is cooled from the outside by relatively cold air.
A considerable reduction in the exhaust gas volume to the afterburning device cannot be achieved with the embodiment according to the aforementioned publication EP 0 723 126 A1. With this embodiment, ambient air is used for the purpose of cooling the open flame of the burner and to prevent cracking of volatile solvent residues. The dwell time of the combustion air mixture in the burner is not long enough to completely burn the volatile solvent components.
The published European Patent Document EP 0 264 637 B1 relates to a continuous-flow dryer for material webs, in particular an offset dryer. The material web is provided, on one or both sides thereof, with blast nozzles acting thereon with heated air, with at least one ventilator which supplies the blast nozzle with ambient air, and with at least one gas-fed heating device for the ambient air and an afterburning device for the ambient air. An inlet slot and an outlet slot are provided for the material web wherein, in the region of at least one of these slots, mixing chambers are provided above and below the plane of material web guidance equipment and the fresh air flowing in via the inlet slot and if need be, the outlet slot, is mixed with hot gases of the afterburning device in these mixing chambers. Outlets are provided to the interior of the dryer, wherein each heating device and afterburning device supplied with gas and ambient air of the dryer are combined into one unit and have a closed combustion chamber in the dryer housing.
According to this embodiment of the aforementioned publication EP 0 264 637 B1, each heating and afterburning device is supplied with gas and ambient air of the dryer, closed combustion chambers being disposed inside the dryer. Through the intermediary of this embodiment, air, instead of exhaust gas, is supplied once more to the heating devices, by which the volume to be supplied to the afterburning devices does not decrease significantly. Consequently, the gas consumption of the afterburning devices likewise cannot be significantly reduced.
The published European Patent Document EP 0 326 227 A1 discloses a dryer for a material web. The dryer, which is provided for an offset printing machine, in particular, includes a housing formed with an inlet slot and an outlet slot for the material web. The housing is equipped with blast nozzles, which are supplied with ambient air from inside the dryer via one or more ventilators. The housing, furthermore, includes a heating device with a burner in order to change the atmosphere inside the dryer. The inside of the housing is divided by a partition into a heating zone and a vaporizing or evaporation zone. The heating device, furthermore, includes another zone from which a portion of the heated air is supplied to the heating zone via an outlet opening, while the remainder of the heated air is conducted back to the ambient air. The atmosphere in the evaporation zone serves as combustion air for the burner of the heating device.
In this embodiment, the entire exhaust gas volume is supplied to an afterburning device, an economical embodiment being proposed by the embodiment according to the invention.
In the MEG company brochure, "MEG Operating Description SDE", page MC 10, dated Jul. 5, 1996, there is disclosed a dryer which is divided into a number of zones. In the first zone, the web temperature increases due to the supply of heated air from the combustion chambers. Thereafter, the evaporation of solvents begins. The evaporation of solvents is finished in the second zone of the dryer, which is separated from the first zone by a vertical partition formed with an opening. The required thermal energy is provided by heated air passing from the first zone into the second zone through the opening formed in the partition. The solvents are blown out via an exhaust gas ventilator, which also transports energy between the zones. This embodiment uses separate exhaust gas and ambient air ventilators because the exhaust gas is conducted directly out of the dryer.